For mustard sauce, in a small bowl combine mayonnaise, yogurt, and mustard.
Cover and refrigerate till serving time.
Rub meat with garlic.
Season with salt and pepper.
Place steak on the cooking grill.
Grill 10 to 12 minutes or till outside is well browned and inside is rare, turning once halfway through grilling time.
Transfer steak to a serving platter.
Keep warm.
Split the bread lengthwise.
Spread margarine on the cut surfaces.
Place, cut side down, on the outside edge of the grill to toast lightly.
To serve, slice steak diagonally across the grain into thin slices.
Pile steak slices on bottom half of the toasted bread.
Pour mustard sauce over beef slices.
Replace top of bread.